1. Technical Field of the Invention
The present invention relates generally to a fuel cell system suitable for vehicles, boats, ships, or portable generators which is equipped with a fuel cell working to convert the energy produced by electrochemical reaction of oxygen and hydrogen into electric power and designed to control a supply pressure of fuel with high accuracy.
2. Background Art
There are known fuel cell systems designed to suck an off-gas discharged from a fuel electrode of a fuel cell using a pump and mix it with a fuel supplied to the fuel cell. The pump is usually implemented by an ejector vacuum pump equipped with an ejector nozzle since it is capable of employing fluid energy of the supplied fuel for power saving.
Usually, the fuel cell systems are required to keep a supplied pressure of fuel at a given value in order to decrease a pressure difference between an oxygen electrode and a fuel electrode, stabilize output of the fuel cell, and purge moisture away from the fuel electrode. The ejector vacuum pump is, however, subjected to variation in pressure of fuel at an outlet thereof (i.e., a supplied pressure of the fuel) and flow rate of recirculated off-gas due to variations in pressure and flow rate of the fuel supplied to the fuel cell. The ejector vacuum pump also suffers from a drawback in that a controllable range of the flow rate of the off-gas is narrow.
Japanese Patent First Publication No. 2001-266922 discloses a fuel cell system which has pressure controlling lines and a plurality of pressure control valves capable of being controlled as a function of pressure in an oxidizing agent supplier.
The above system, however, has drawbacks in that the structure made up of the pressure control valves and the bypass lines is complex, control of the pressure of fuel supplied to the fuel cell depends upon the supplied pressure of the oxidizing agent, thereby making it difficult to meet a high-accuracy fuel supply control requirement, and a variation in supplied pressure of the oxidizing agent may result in a variation in supplied pressure of the fuel (i.e., hunting), which leads to instability of operation of the fuel cell.
Further, the moisture contained in the off-gas may freeze near the ejector nozzle in low-temperature environments, thereby resulting in changes in area of an outlet of the nozzle and state of a wall surface of the nozzle, which may cause a disturbance of control of flow rate of the off-gas.